The research objective of this project is to investigate the interactions of adeno-associated virus (AAV) with its host cell. The underlying hypothesis is that by understanding these interactions as they apply to the biology of the virus we can contribute to the use of AAV vectors for gene therapy. The Unit?s efforts have focused on the characterization of two new AAV serotypes, AAV4 and AAV5. These new viral isolates are being studied both as natural mutations of other serotypes for understanding the biology of this genus of virus and because of their unique cell tropism, as novel vectors for gene transfer. AAVs are dependent viruses and require other viruses and cellular products for vital functions. This dependency coupled with the stability of the AAV2 particles has made AAV2 an attractive vector for gene therapy. AAV2 vectors can be maintained stably in the host cell, allowing long term expression of the transduced genes. However, AAV2 demonstrates a wide variability in transduction efficiency of different cell types, and poor transduction efficiency upon repeat administration of the vector. While some of these limitations may be overcome as our understanding of the basic biology of AAV increases, others may require employing different naturally occurring isolates of AAV serotypes. To date 6 primate isolates have been cloned and are referred to as AAV1-6. Our previous work with AAV4 and AAV5 has demonstrated distinct tropisms for these viruses compared to AAV2. While heparin sulfate proteoglycans are important in AAV2 binding and transduction, AAV4 and AAV5 interact with the cell surface by a distinct mechanism. Our current work indicates these serotypes bind the cell surface via another charged carbohydrate, sialic acid. Treatment with neuraminidase, competition with soluble sialic acid conjugates or resialyation experiments with specific sialyltransferases indicate that both serotypes require 2-3 sialic acid for efficient binding and transduction. However in keeping with their different cell tropism AAV4 requires O-linked 2-3 sialyltransferase and AAV5 requires 2-3 N-linked.